There are polymer foams used for insulation and packaging applications with good performance to price ratio. But because these forms are petroleum-derived, there is increasing environmental and consumer demand for using biofoams—biologically derived foams—for some applications.
PLA is a ‘green plastic’ being bio-derived and bio-degradable. A useful blowing agent, carbon dioxide, is a ‘green’ blowing agent because it has no ozone depletion potential and a tiny global warming potential. A number of attempts have been made to develop a process for foaming PLA in order to provide a green alternative to materials such as polystyrene foam (expanded polystyrene/EPS). Because of a small processing window and the theological properties of the PLA polymer melt, many of these processes were unsuccessful or unsatisfactory. The processes that were to some extent successful often required complex processes and/or additives such as nucleating agents to improve the foaming and the fusing parts of the process. See for example United States patent publication US 2006-0167122 that reports that use of a nucleating agent is necessary.
There are a number of reported processes that use carbon dioxide as a blowing agent including uses in relation to PLA. Reported PLA foaming and moulding processes using PLA resin beads impregnated with carbon dioxide generally involve impregnating the beads with gaseous or supercritical CO2, pre-expanding the impregnated beads, resting and sometimes treating the pre-expanded beads, before re-impregnating them with more CO2 or another blowing agent and further expanding and fusing them in a mould. See for example European patent application EP 1 378 538 that reports use of blends comprising predominantly crystalline PLA that require re-impregnation with blowing agent before moulding.
It is an object of the present invention to provide a simple method of foaming PLA resin beads or to at least provide the public with a useful choice.